What is the interface upgrade (optional interface) pattern in Go?

// Basic interface — all implementations must satisfy
type Writer interface {
    Write(p []byte) (int, error)
}

// Enhanced optional interface
type StringWriter interface {
    WriteString(s string) (int, error)
}

// Function accepting Writer — works with any Writer
// but uses WriteString if available (avoids []byte conversion)
func writeString(w Writer, s string) (int, error) {
    // Interface upgrade: check if w also supports WriteString
    if sw, ok := w.(StringWriter); ok {
        return sw.WriteString(s)         // faster path
    }
    return w.Write([]byte(s))            // fallback
}

// This is exactly how bufio.Writer in the stdlib does it:
// bufio.Writer.WriteString checks if the underlying writer
// implements WriteString and avoids the []byte allocation

// Another real example: http.Flusher
func streamResponse(w http.ResponseWriter, data []byte) {
    w.Write(data)
    // Check if the ResponseWriter supports streaming flush
    if flusher, ok := w.(http.Flusher); ok {
        flusher.Flush() // sends bytes to client immediately
    }
}

// http.Hijacker — upgrade ResponseWriter to take over TCP connection
// http.CloseNotifier — deprecated, but same pattern
// io.WriterTo, io.ReaderFrom — upgrade for efficient copy